1. Field of the Invention
This invention relates to a mobile broadcast receiving apparatus which selectively receives either broadcast radio waves from a geostationary satellite or radio waves retransmitted from a gap filler according to reception levels thereof, and a control method for such apparatus. In particular, this invention relates to a mobile broadcast receiving apparatus which is capable of rapid recovery from image distortion and reducing the power consumption by appropriately controlling the switching of the dynamic range of a low noise amplifier (LNA), the time constant of an automatic gain control circuit (AGC), and ON and OFF of antenna diversity, and a control method for such apparatus.
2. Description of the Related Art
Mobile broadcasting is one of new satellite broadcast services, which enables a user to enjoy sharp and clear video reception even in a moving vehicle or open air.
In the mobile broadcasting, broadcast radio waves are basically received from a geostationary satellite to be reproduced into video or the like. On the other hand, in order to enable reception at locations where direct reception of broadcast radio waves from the geostationary satellite is impossible, terrestrial retransmitting equipment, which is called “gap filler”, is installed to receive broadcast radio waves from the geostationary satellite and to retransmit the received radio waves.
The gap filler is installed at a location where direct reception of broadcast radio waves from the geostationary satellite is impossible, for example in the shade of a building.
The provision of a gap filler enables a mobile broadcast receiving apparatus to selectively receive either broadcast radio waves transmitted directly from the geostationary satellite or radio waves retransmitted from the gap filler. This makes it possible for a user to enjoy sharp and clear video reception even in a moving vehicle or in open air.
A satellite-mobile broadcasting system disclosed in Japanese Patent Application Laid-Open No. 2004-312349 is one of such apparatuses.
In the mobile broadcasting, CDM (code division multiplexing) waves are transmitted from a geostationary satellite. A reception side receives the CDM waves by an antenna, and sequentially processes the received signal by an RF module including a low noise amplifier (LNA) and an automatic gain control circuit (AGC), a demodulation module, and a decoding module to reproduce and store the received video and audio signals.
On the other hand, the gap filler is installed, for example, on a rooftop of a building on the ground so as to cover the radio silent zone of the geostationary satellite. The gap filler receives broadcast radio waves from the satellite, amplifies the amplitude thereof, and retransmits the radio waves.
In the mobile broadcast receiving apparatus which selectively receives either broadcast radio waves transmitted directly from the geostationary satellite or radio waves retransmitted from the gap filler, the selective reception is conducted according to reception levels of the broadcast radio waves from the geostationary satellite and the retransmitted radio waves from the gap filler. In the real field, there occurs a difference between the reception level on the ground of the broadcast radio waves from the geostationary satellite which is about 36,000 Km up in the air and the reception level of the radio waves retransmitted from the gap filler which is installed on the ground.
A user who carries the mobile broadcast receiving apparatus is unable to come close to the geostationary satellite, but able to do so to the gap filler. Therefore, the reception level of the radio waves retransmitted from the gap filler (gap filler waves) varies more significantly.
Specifically, the reception level of the broadcast radio waves from the geostationary satellite (satellite waves) is always low, while the reception level of the gap filler waves becomes higher as the user comes closer to the gap filler.
Consideration is given, for example, to the case in which the user carrying the mobile broadcast receiving apparatus moves through a group of buildings consisting of buildings 9-1 to 9-8, as shown in FIG. 9.
It is assumed here that a gap filler 91 is installed on the rooftop of the building 9-1.
Since the geostationary satellite 90 is located far enough from the building group, the satellite waves from the geostationary satellite 90 reach the building group from a fixed direction, and the reception level of the satellite waves is always low.
In contrast, the reception level of gap filler waves from the gap filler 91 varies in accordance with the distance between the mobile broadcast receiving apparatus and the gap filler 91.
FIG. 10 shows an example of reception levels at the mobile broadcast receiving apparatus when the user carrying the apparatus moves in the field with the distributed electric field strength shown in FIG. 9 from point A, to point B, and to point C.
While the user is moving from point A to point B, the mobile broadcast receiving apparatus carried by the user is separated from the gap filler 91 by a long distance. Therefore, the reception level of the gap filler waves is on average lower than that of the satellite waves (region R1).
In contrast, when the user moves from point B to point C, the mobile broadcast receiving apparatus carried by the user is brought closer to the gap filler 91, and hence the reception level of the gap filler waves increases rapidly. Therefore, the reception level of the gap filler waves is on average higher than that of the satellite waves (region R3).
When in the vicinity of point B, the reception level of the gap filler waves becomes substantially equal to that of the satellite waves (region R2).
In the regions R1 and region R3, there occurs fading or shadowing due to effects of objects on the ground, which fluctuates the reception level. However, the possibility is low that the relationship in reception level between the gap filler waves and the satellite waves is reversed.
In the region R3, the reception level of the gap filler waves is relatively stable since the distance between the mobile broadcast receiving apparatus and the gap filler 91 is small.
In contrast, in the region R2, the relationship in reception level between the gap filler waves and the satellite waves changes frequently.
The range of level variation of the satellite waves from the geostationary satellite is small, whereas the range of level variation of the gap filler waves from the gap filler is large. Therefore, the dynamic range needs to be switched in a low noise amplifier (LNA) which low-noise amplifies the satellite waves and gap filler waves in common.
However, the reception level the satellite waves may temporarily increase due to noises or the like in the region R1 where the reception level of the gap filler waves is on average lower than that of the satellite waves. If the reception level of the satellite waves exceeds the switching level set for the dynamic range of the low-noise amplifier circuit as shown in FIG. 10, the dynamic range of the low-noise amplifier circuit is thereupon switched. This may eventually hinder appropriate demodulation of the waves.
In the region 2 where the reception level of the gap filler waves on average is substantially equal to the reception level of the satellite waves, for example, rapid switching between the gap filler waves and the satellite waves may become impossible due to the time constant of the automatic gain control circuit (AGC).
When antenna diversity is employed, for example, the antenna diversity will be used also in the region R3 where the use of the antenna diversity is unnecessary since the reception level of the gap filler waves is on average higher than that of the satellite waves. This will pose a problem of wasteful consumption of power.
It is therefore an object of this invention to provide a mobile broadcast receiving apparatus which is capable of stably receiving mobile broadcasting even if rapid variation occurs in the reception level, and yet capable of reducing power consumption, and a control method for such apparatus.